1. Field of the Invention
The present invention relates to the collection of solar energy, and more particularly to a solar collector and an apparatus and method for making a solar collector that concentrates energy (predominantly in the form of incident light) from the sun.
2. Description of the Related Art
Solar collectors gather energy from the sun. The gathered energy may be used for various applications, including generating electricity, heating water and producing motion (in a sterling engine or equivalent heat engine).
A particular type of solar collector is a solar concentrator. Solar concentrators increase the energy intensity of sunlight by focusing light rays to a defined location (generally denoted as the focal location of the collector). The collection capacity of a solar concentrator is defined by the reflective surface area available to concentrate incident light (the collection surface area), although the specific configuration of the collection surface and other considerations influence the overall concentration efficiency (the amount of energy concentrated for a given collection surface area).
Solar concentrators focus sunlight that is incident on the collection surface to a surface of smaller area at the focal location. By concentrating light from a larger area (the collection surface) to a smaller area (the focal location), the solar concentrator increases the irradiance (a measure of the collected lights energy intensity, measured as the energy per unit area) of the light. The concentration factor of a solar concentrator is the ratio of the collection surface area to the focal location area. A larger concentration factor indicates greater light intensity at the focal location for a given collection surface area.
The energy concentrated at the focal location is capable of generating extremely high temperatures. This high energy intensity makes solar concentrators particularly suitable for heating applications. Solar concentrators can also reduce the capital cost of converting energy into electricity, as a smaller photovoltaic cell area is required to capture the light of the collection surface (although the cell may require greater temperature resistance).
Conventional solar concentrators may be anchored in a stationary position or mounted to a tracking mechanism that follows the movement of the sun across the sky. The mounting arrangement of the concentrator influences the collection surface design of the concentrator. Stationary concentrators focus incident light to the same focal location independent of the sun's position. However, the shape of the collection surface employed in stationary concentrators generally produces a lower concentration factor and reduced efficiency compared with tracking concentrators, a result of having to accommodate a greater range of incident light angles. Conversely, the shape of the collection surface employed in tracking concentrators can be optimized for a particular angle of light incidence at the expense of increased capital expenditure associated with the tracking mechanism.
Conventionally, tracking solar concentrators are fabricated with large collection surface areas. These dishes are capable of concentrating large quantities of solar energy into a small space at the focal location. It is not uncommon for parabolic solar concentrating dishes to have diameters in excess of 6 ft to enhance the sunlight gathering capabilities of the collection surface. Large solar concentrators have several advantages. Primarily, the number of transducers or other energy conversion mechanisms required to utilize the concentrated sunlight for a given surface is reduced. This is a common motivator for using solar concentrators, as the transducer contributes a significant cost to the overall solar collection system. The number of transducers required to utilize the collected energy for a given surface area is directly related to the number of solar concentrating dishes employed, as each dish concentrates energy to a unique focal location.
Another advantage is that a reduced number of tracking mechanisms are required to orientate the dish of a tracking concentrator with respect to the sun. Tracking mechanisms, like transducers, introduce substantial cost to the solar collection system.
However, there are also several disadvantages of large surface area solar concentrators. Primarily, the large collection surfaces are difficult to fabricate. The size of each dish prevents standardized machining and consequently most dishes are hand shaped by a specialist. The collection surface is commonly formed from several large panels that are individually shaped and subsequently joined together, a process that introduces inaccuracies that affect the reflection characteristics of the dish. The inaccuracies can usually be attributed to inevitable shaping variations, the accumulation of tolerances over the entire collection surface and irregularities at the joints between adjacent panels forming the surface.
The overall curvature variation for a collection surface comprising individually shaped panels is an accumulation of tolerances from the individual panels and the joints between adjacent panels. Commonly, each panel is shaped to a desired tolerance. A tolerance is also allocated for the joints between adjacent panels when the collection surface is formed. The overall tolerance for the collection surface is then a combination of the tolerances for the individual components, making accurate surface curvatures increasing difficult to obtain. Significant surface irregularities can occur in collection surfaces formed from panels even when the individual panels are within desired tolerance limits.
The largest curvature irregularities in panel collection surfaces typically occur at the joint between adjacent panels. Joint irregularities can create rapid transitions in surface curvature (sometimes characterized by surface discontinuities) that disrupt reflected light and reduce the efficiency of the concentrator.
The large size of panel surface concentrating dishes can also translate into significant weight. It is not uncommon for large concentrating dishes to weigh in excess of 500 lbs. The excessive weight of large panel dishes necessitates greater support facilitates, which can increase the initial cost of the concentrating system. Weight is particularly important when considering tracking solar dishes, as the tracking mechanism must incorporated larger actuators to offset the mass of the dish. Another complication is wind shear, which increases with surface area. To address wind shear, both the dish and the supporting structure need to be adequately reinforced.